MRI is an imaging method which excites nuclear spin of an object set in a static magnetic field with a RF (radio frequency) signal having the Larmor frequency magnetically and reconstructs an image based on MR (magnetic resonance) signals generated due to the excitation.
In MRI, a center frequency of a RF pulse is set to the resonance frequency of water which is a target of signal acquisition. Therefore, a pre-scan is performed for acquiring a frequency spectrum of signals prior to an imaging scan in order to detect peaks of resonance frequencies mutually different between materials. The resonance frequency of water shifts from that of fat relatively by about 3.5 ppm (parts per million). Therefore, a fat saturation pulse is applied in a pre-scan ordinarily so that a peak does not appear at the frequency corresponding to fat in a frequency spectrum. The fat saturation pulse is a RF pulse for suppressing signals from fat.
Then, a peak is detected from the frequency spectrum having been acquired by the pre-scan, and subsequently, a frequency corresponding to the detected peak is set to the center frequency of RF pulses as the resonance frequency of water.
However, if fat component abounds in an imaging region like a case of imaging a chest part especially, fat suppression becomes insufficient. Consequently, a shape of a frequency spectrum is disturbed and it becomes difficult to detect a frequency corresponding to a water peak with a high degree of accuracy. Sometimes a peak larger than that corresponding to water appears at the resonance frequency of fat. Hence, there is the possibility of recognizing the resonance frequency of fat as that of water erroneously.
On the other hand, detection of the resonance frequency of water and set up of the center frequency of RF pulses are desired to be automated. Therefore, it is important to detect the resonance frequency of water with a sufficient accuracy without fail not only in a case of detecting the resonance frequency of water by visual check of a frequency spectrum by a user to match the center frequency of RF pulses with the resonance frequency of water but also in a case of automatically detecting the resonance frequency of water and setting the center frequency of RF pulses.
The same concerns apply to a case of setting the center frequency of RF pulses to a resonance frequency of a material other than water for molecular imaging which generates MR signals from a specific material for imaging.
An object of the present invention is to provide a magnetic resonance imaging apparatus and a magnetic resonance imaging method which can detect a resonance frequency of a material, such as water, which is a target of signal acquisition with higher accuracy.